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| United States Patent |
6,001,756
|
|
Takahashi
, et al.
|
December 14, 1999
|
Process for making a silicon carbide sintered body
Abstract
A silicon carbide sintered body according to the present invention is a
silicon carbide sintered body having a density of 2.9 g/cm.sup.3 or
higher, obtained by means of hot pressing a mixture of silicon carbide
powder and a non-metal-based sintering additive such as an organic
compound which produces carbon upon heating at a temperature of
2,000.degree. C. to 2,400.degree. C. and under a pressure of 300 to 700
kgf/cm.sup.2 in a non-oxidizing atmosphere. It is preferable that the
silicon carbide powder have an average particle diameter of from 0.01 to
10 .mu.m and that the non-metal sintering additive be a resol type phenol
resin. The present invention is to provide a silicon carbide sintered body
of high quality which has a high density, a high purity, and a high
electrical conductivity and which is useful for semiconductor
manufacturing industry.
| Inventors:
|
Takahashi; Yoshitomo (Fujisawa, JP);
Wada; Hiroaki (Kawasaki, JP);
Miyamoto; Taro (Yokohama, JP)
|
| Assignee:
|
Bridgestone Corporation (Tokyo, JP)
|
| Appl. No.:
|
853719 |
| Filed:
|
May 9, 1997 |
Foreign Application Priority Data
| Feb 29, 1996[JP] | 8-43748 |
| Jun 17, 1996[JP] | 8-155670 |
| Current U.S. Class: |
501/90; 264/604; 264/682 |
| Intern'l Class: |
C04B 035/569 |
| Field of Search: |
501/90
264/604,682
|
References Cited
U.S. Patent Documents
| 4524138 | Jun., 1985 | Schwetz et al. | 501/90.
|
| 4564601 | Jan., 1986 | Kriegesmann et al. | 501/90.
|
| 4742029 | May., 1988 | Kurachi et al. | 501/90.
|
| 4925815 | May., 1990 | Tani et al. | 501/90.
|
| 4980104 | Dec., 1990 | Kawasaki | 501/90.
|
| 5093039 | Mar., 1992 | Kijima et al.
| |
| 5094985 | Mar., 1992 | Kijima et al.
| |
| 5182059 | Jan., 1993 | Kawasaki et al. | 501/90.
|
| 5470806 | Nov., 1995 | Kristic et al. | 501/90.
|
| 5543368 | Aug., 1996 | Talbert et al. | 501/90.
|
| 5656213 | Aug., 1997 | Sakaguchi et al. | 501/90.
|
| 5863325 | Jan., 1999 | Kanemoto et al. | 501/88.
|
| Foreign Patent Documents |
| 60-108370 | Jun., 1985 | JP.
| |
| 2-199064 | Aug., 1990 | JP.
| |
| 7-241856 | Sep., 1995 | JP.
| |
| 2 301 349 | Feb., 1996 | GB.
| |
Primary Examiner: Group; Karl
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A process for making a silicon carbide sintered body, comprising:
providing a silicon carbide powder consisting essentially of silicon
carbide particles that have an average particle diameter of 0.01 to 10
.mu.m and are formed by the same powder making process; and
a sintering step in which a mixture of the silicon carbide powder and a
sintering additive that consists essentially of a non-metal-based
sintering additive is hot-pressed at a temperature of 2,000.degree. C. to
2,400.degree. C. and under a pressure of 300 to 700 kgf/cm.sup.2 in a
non-oxidizing atmosphere to obtain a silicon carbide sintered body having
a density of 2.9 g/cm.sup.3 or higher.
2. A process for making a silicon carbide sintered body as claimed in claim
1, further comprising a molding step in which a homogenous mixture of the
silicon carbide powder and the non-metal-based sintering additive is
placed in a mold and heated in the mold at a temperature ranging from
80.degree. C. to 300.degree. C. for 5-60 minutes to obtain a molded body,
and thereafter, the resultant molded body is subjected to the sintering
step.
3. A process for making a silicon carbide sintered body as claimed in claim
1, wherein the non-metal-based sintering additive is an organic compound
which produces carbon upon heating.
4. A process for making a silicon carbide sintered body as claimed in claim
3, further comprising a step of coating an organic compound that produces
carbon upon heating on the surface of the silicon carbide powder by mixing
the non-metal-based sintering additive by itself, in the form of a
solution in a solvent, or in the form of a dispersion in a dispersion
medium, with the silicon carbide powder.
5. A process for making a silicon carbide sintered body as claimed in claim
3, wherein the mixture comprises an effective amount of the
non-metal-based sintering additive to produce more carbon than a
stoichiometric amount which is sufficient to reduce silicon oxide present
on the surface of the silicon carbide powder covered.
6. A process for making a silicon carbide sintered body as claimed in claim
3, wherein the organic compound which produces carbon upon heating is a
resol type phenol resin.
7. A process for making a silicon carbide sintered body as claimed in claim
5, wherein the organic compound which produces carbon upon heating is a
resol type phenol resin.
8. A process for making a silicon carbide sintered body as claimed in claim
1, wherein the non-metal-based sintering additive comprises silicon
carbide powder covered with an organic compound that produces carbon upon
heating.
9. A process for making a silicon carbide sintered body as claimed in claim
1, wherein the powder making process for making the silicon carbide powder
comprises:
solidifying a mixture of (1) a liquid silicon compound, (2) a liquid
organic compound that produces carbon upon heating, and (3) a
polymerization catalyst or a cross-linking catalyst, so as to obtain a
solid product; and
heating the solid product in a non-oxidizing atmosphere to carbonize the
solid product and sintering the solid product in a non-oxidizing
atmosphere.
10. A process for making a silicon carbide sintered body as claimed in
claim 9, wherein the liquid silicon compound is ethyl silicate.
11. A process for making a silicon carbide sintered body as claimed in
claim 9, wherein the non-metal-based sintering additive is a resol type
phenol resin.
12. A process for making a silicon carbide sintered body, comprising:
providing a silicon carbide powder consisting essentially of silicon
carbide particles that have an average particle diameter of 0.01 to 10
.mu.m and are formed by the same powder making process;
mixing the silicon carbide powder with a sintering additive that consists
essentially of a non-metal-based sintering additive to form a mixture; and
hot pressing the mixture in a non-oxidizing atmosphere to obtain a silicon
carbide sintered body having a density of 2.9 g/cm.sup.3 or higher.
13. A process for making a silicon carbide sintered body as claimed in
claim 12, wherein the mixture is hot pressed at a temperature of
2,000.degree. C. to 2,400.degree. C. and under a pressure of 300 to 700
kgf/cm.sup.2.
14. A process for making a silicon carbide sintered body as claimed in
claim 12, wherein the non-metal based sintering aid consists essentially
of an organic compound that produces carbon upon heating.
15. A process for making a silicon carbide sintered body as claimed in
claim 12, wherein the powder making process for making the silicon carbide
powder comprises:
solidifying a mixture of (1) a liquid silicon compound, (2) a liquid
organic compound that produces carbon upon heating, and (3) a
polymerization catalyst or a cross-linking catalyst, so as to obtain a
solid product; and
heating the solid product in a non-oxidizing atmosphere to carbonize the
solid product and sintering the solid product in a non-oxidizing
atmosphere.
16. A process for making a silicon carbide sintered body as claimed in
claim 12, wherein the silicon carbide sintered body has a total impurity
content of 5 ppm or less.
17. A process for making a silicon carbide sintered body, comprising:
providing a silicon carbide powder consisting essentially of silicon
carbide particles that have an average particle diameter of 0.01 to 10
.mu.m and are formed by the same powder making process;
mixing the silicon carbide powder with a non-metal-based sintering additive
to form a mixture; and
hot pressing the mixture in a non-oxidizing atmosphere to obtain a silicon
carbide sintered body that has a density of 2.9 g/cm.sup.3 or higher and
is free of boron.
18. A process for making a silicon carbide sintered body as claimed in
claim 17, wherein the mixture is hot pressed at a temperature of
2,000.degree. C. to 2,400.degree. C. and under a pressure of 300 to 700
kgf/cm.sup.2.
19. A process for making a silicon carbide sintered body as claimed in
claim 17, wherein the non-metal based sintering aid consists essentially
of an organic compound that produces carbon upon heating.
20. A process for making a silicon carbide sintered body as claimed in
claim 17, wherein the powder making process for making the silicon carbide
powder comprises:
solidifying a mixture of (1) a liquid silicon compound, (2) a liquid
organic compound that produces carbon upon heating, and (3) a
polymerization catalyst or a cross-linking catalyst, so as to obtain a
solid product; and
heating the solid product in a non-oxidizing atmosphere to carbonize the
solid product and sintering the solid product in a non-oxidizing
atmosphere.
21. A process for making a silicon carbide sintered body as claimed in
claim 17, wherein the silicon carbide sintered body has a total impurity
content of 5 ppm or less.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon carbide sintered body and a
process for making the same and, more particularly, to a silicon carbide
sintered body having a high density useful for structural parts and
components of semiconductor manufacturing equipment, electronic
information equipment, vacuum devices, and the like.
2. Description of the Related Art
Silicon carbide is a highly covalent material and has found various
applications taking advantage of its properties of, for example, strength
at a high-temperature, heat resistance, wear resistance and chemical
resistance. Recently, there have been demands in the fields of materials
for electrical information equipment and semiconductor manufacturing
toward a high heat-resistance, and a highly dense and highly pure silicon
carbide sintered body without heat distortion and due to heat
deterioration when washed with chemicals such as hydrofluoric acid, as in
conventional parts and components made of quartz with increase in wafer
processing temperature, wafer diameter, and processing unit.
As mentioned above, the silicon carbide is highly covalent and is thus
difficult to sinter. Dense silicon carbide sintered bodies are known to be
produced by means of hot pressing, reaction sintering, and atmospheric
sintering.
The hot pressing is a method in which silicon carbide is sintered under a
high pressure and has been studied with a variety of metal sintering
additives after this technique was reported (J. Am. Ceram. Soc., 39(11),
386-389 (1956)) with aluminum added as a metal sintering additive. A
highly conductive and electrically insulating sintered body was developed
in 1980 that was sintered by the hot pressing by adding BeO("Silicon
Carbide Ceramics", pages 327-343 published by Uchida Rokakuho (1988)).
The reaction sintering involves in the following steps of: (1) mixing raw
materials (i.e., mixing silicon carbide powder and carbon powder), (2)
molding fabricating, (3) reaction sintering, and (4) post-processing, if
necessary. This technique involves silicification of the carbon particles
which have already been molded in the reaction sintering step (3) above,
which provides an advantage of allowing sintered bodies of less variation
in dimension without any sintering additives. Accordingly, this technique
provides an easier operation to produce a sintered body of a higher purity
and has been used for production of parts and components for
semiconductors. However, the sintered bodies obtained by means of this
technique contains non-reacted metal silicon so that their applications
are restricted when parts and jigs are used in the fields where the heat
resistance, the chemical resistance and a high strength are required.
The atmospheric sintering process is a technique characterized by using
sintering additives for sintering the silicon carbide and was proposed by
S. Prochazka in "Ceramics for High Performance Applications" in 1974, on
page 239. This technique allows production of a high dense structural
member having a high strength at a high temperature. As a result, studies
of the silicon carbide have been developed. The sintering additive used is
a combination of a metal-based sintering additive comprising a metal such
as boron, aluminum and beryllium or a compound thereof and a carbon-based
sintering additive such as carbon black and graphite. The metal-based
sintering additive has essential effects of, for example, reduction of
surface energy at a grain boundary due to local deposit of boron onto the
boundary, enhancement of diffusion of carbon-boron substances on the grain
boundary, and suppression of surface diffusion thereof, for the boron
which is used as the optimum sintering additive. The carbon-based
sintering additive is considered to have an effect of removing an oxide
layer on the surface of silicon carbide particles. Details, however, have
still remained unknown.
Metal contaminants are eluded when the metal-based sintering additive used
is exposed to a high temperature or is subjected to washing with
chemicals. The resultant sintered bodies are thus not suitable for the
application to the areas of the semiconductor manufacturing equipment.
In order to overcome the above mentioned problems, Japanese Patent
Application Laid-Open (JP-A) No. 60-108370 proposes a process for making a
dense sintered body by hot press process without adding a sintering
additive, using a special ultra-fine powder of silicon carbide obtained
through heat decomposition of a silane compound. However, there is no
clear description on properties of the resultant sintered bodies. In this
connection, "Silicon Carbide Ceramics", published by Uchida Rokakuho, in
1988, on page 89, describes that it is essential to add boron as a
metal-based sintering additive even by using the powder obtained according
to this technique.
As an improved hot pressing, Japanese Patent Application Laid-Open (JP-A)
No. 2-199064 proposes a process for making a dense sintered body without
any additives by the hot press method, using ultra-fine silicon carbide
powder synthesized by means of CVD plasma technique. However, impurities
such as iron are contained in an amount of several ppm or more even in the
process described in this application. This is not a satisfactory level of
impurities. The ultra-fine silicon carbide powder used for this system as
the sintering additive has an average particle diameter of 30 nm. Such
ultra-fine powder is relatively expensive and should be treated with
significant care for preventing oxidation on the surface thereof. In light
of the above, the process disclosed in the above application is far from a
solution to the problems to date.
It is difficult for the conventional processes to obtain a high dense
silicon carbide sintered body, containing less or no impurities, which is
suitable for being used for parts and components of semiconductor
manufacturing equipment and electronic information equipment. Furthermore,
there is no such a sintered body available on the market.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for making a
silicon carbide sintered body of high quality, without using any special
material, which has a high density and a high purity and which is suitable
for being used for parts and components of semiconductor manufacturing
equipment, and to provide a silicon carbide sintered body of high quality
obtained by such process in which the sintered body has a high density, a
high purity, a high electrical conductivity, and a high thermal
conductivity and is useful in various fields including industries of
semiconductor and electronic information equipment.
The present inventors had made extensive studies on the above-described
history of development of sintering processes. As a result, it has found
that a highly dense and highly pure silicon carbide sintered body can be
obtained by means of previously placing a non-metal-based sintering
additive represented by carbon in an adequate amount on silicon carbide
powder and combining hot pressing under specific conditions. The present
invention was thus completed.
More specifically, according to a first aspect of the present invention, a
silicon carbide sintered body according to the present invention is
characterized by being obtained by sintering a mixture of silicon carbide
powder and a non-metal-based sintering additive, wherein the sintered body
has a density of 2.9 g/cm.sup.3 or higher.
The non-metal-based sintering additive of the present invention is
preferably an organic compound that produces carbon upon heating. It is
more preferable that the non-metal-based sintering additive be a resol
type phenol resin.
In the present invention, it is preferable that the surface of the silicon
carbide powder be covered with the non-metal-based sintering additive.
It is preferable that the silicon carbide sintered body of the present
invention contains carbon in an amount larger than 30% by weight but not
larger than 40% by weight.
According to a second aspect of the present invention, a process for making
a silicon carbide sintered body according to the present invention is
characterized by comprising a sintering step in which a mixture of silicon
carbide powder and a non-metal-based sintering additive is hot-pressed at
a temperature of 2,000.degree. C. to 2,400.degree. C. and a pressure of
300 to 700 kgf/cm.sup.2 in a non-oxidizing atmosphere to obtain a silicon
carbide sintered body having a density of 2.9 g/cm.sup.3 or higher.
In the process for making a silicon carbide sintered body according to the
present invention, it is preferable that the non-metal-based sintering
additive be an organic compound that produces carbon upon heating. It is
more preferable that the non-metal-based sintering additive be a resol
type phenol resin.
In the process for making a silicon carbide sintered body according to the
present invention, the homogenous mixture of the silicon carbide powder
and a non-metal-based sintering additive may be placed in a mold and
heated at a temperature ranging from 80.degree. C. to 300.degree. C. for
5-60 minutes to obtain a molded body, and thereafter, the resultant molded
body is subjected to the above mentioned sintering step.
The process for making a silicon carbide sintered body according to the
present invention preferably comprises a step of coating a non-metal-based
sintering additive on the surface of the silicon carbide powder by means
of mixing the sintering additive as such or in the form of solution in a
solvent or of dispersion in a dispersion medium with the silicon carbide
powder.
In the process for making a silicon carbide sintered body according to the
present invention, it is preferable that an amount of a non-metal-based
sintering additive be an amount to produce more carbon than stoichiometric
amount which is enough to reduce silicon oxide present on the surface of
the silicon carbide powder covered.
It is preferable that the silicon carbide powder used in the process for
making a silicon carbide sintered body according to the present invention
have an average particle diameter of from 0.01 to 10 .mu.m.
In the process for making a silicon carbide sintered body according to the
present invention, it is preferable that the silicon carbide powder be
obtained through a production process comprising a solidification step for
solidifying a homogenous mixture of (1) a liquid silicon compound, (2) a
liquid organic compound that produces carbon upon heating, and (3) a
polymerization catalyst or a cross-linking catalyst to obtain a solid
product; and a baking step for heating the resultant solid product in a
non-oxidizing atmosphere to carbonize the solid product and sintering it
in a non-oxidizing atmosphere. It is more preferable that the liquid
silicon compound be ethyl silicate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is described more in detail below.
Silicon carbide powder used as a raw material for the silicon carbide
sintering body according to the present invention may be an .alpha.-type,
a .beta.-type, an amorphous type, or a mixture thereof. In particular, the
.beta.-type silicon carbide powder can advantageously be used. There is no
specific limitation on the grade of the .beta.-type silicon carbide
powder. For example, a commercially available .beta.-type silicon carbide
powder may be used. The particle diameter of the silicon carbide powder is
preferably as small as possible from the viewpoint of increasing the
density. It is preferable that the particle diameter thereof be from 0.01
to 10 .mu.m, and more preferably from 0.05 to 1 .mu.m. The particle
diameter of smaller than 0.01 .mu.m results in difficulties in handling in
subsequent steps including weighing and mixing. On the other hand, it is
not preferable that the particle diameter is larger than 10 .mu.m so that
the specific surface area thereof becomes small, i.e., the contact area
with adjacent particles becomes small to make densification difficult.
With respect to this, such silicon carbide powder may suitably be used that
has a particle diameter of 0.05 to 1 .mu.m, a specific surface area of 5
m.sup.2 /g or larger, a free carbon content of 1% or less, and an oxygen
content of 1% or less. In addition, the particle size distribution of the
silicon carbide powder used is not limited to specific one. Silicon
carbide powder having two or more size distribution peaks may be used in
view of a high packing density of the powder and a high reactivity of the
silicon carbide powder during the manufacturing process of the silicon
carbide sintering body.
A silicon carbide powder of high purity may be used as a raw material
silicon carbide powder in order to obtain a silicon carbide sintered body
of high purity.
The silicon carbide powder of high purity may be obtained through a
production process comprising a sintering step for sintering, in a
non-oxidizing atmosphere, a solid product obtained from a homogenous
mixture of, for example, a silicon source containing at least one liquid
silicon compound, a carbon source containing at least one liquid organic
compound that produces carbon upon heating, and a polymerization or
cross-linking catalyst. The liquid silicon compound may be used together
with a solid silicon compound.
The silicon compound used for producing the silicon carbide powder of high
purity (hereinafter, conveniently referred to as a silicon source) may be
a combination of a liquid one and a solid one. However, at least one
silicon compound should be in a liquid form. Examples of a liquid silicon
compound include alkoxysilane (mono-, di-, tri-, tetra-) and polymers of
tetraalkoxysilane. Among alkoxysilane, tetraalkoxysilane is advantageously
used. More specifically, methoxysilane, ethoxysilane, propoxysilane, and
butoxysilane may be used adequately. However, it is preferable to use
ethoxysilane from the viewpoint of handling. Examples of the polymers of
tetraalkoxysilane include low molecular weight polymers (oligomers) having
a degree of polymerization of 2 to 15, and liquid polymers of silicic acid
having a higher degree of polymerization. Examples of the solid silicon
compound which maybe combined with the liquid one include silicon oxide.
It is noted that the silicon oxide used herein includes SiO, silica sol
(colloidal ultra-fine silica containing solution, which contains an OH or
alkoxy group therein) and silicon dioxide (silica gel, fine silica, quartz
powder).
Of these silicon sources, it is suitable to use an oligomer of
tetraethoxysilane or a mixture of an oligomer of tetraethoxysilane and
ultra-fine powder of silica in light of homogeneity and handling. These
silicon sources used are highly pure substances and preferably contain
impurities in an amount of not larger than 20 ppm, and more preferably 5
ppm, at an initial stage.
The organic compound that produces carbon upon heating, which is used for
the production of the highly pure silicon carbide powder, may be in a
liquid form or a mixture of liquid and solid forms. It is preferable that
the organic compound be the one that has a higher residual carbon ratio
and is to be polymerized or cross-linked by means of a catalyst or heat.
More specifically, examples of such organic compound include monomers and
prepolymers of phenol resins, furan resins, other resins such as
polyimide, polyurethane, and polyvinyl alcohol. In addition, liquid
compounds of cellulose, sucrose, pitch and tar may be used. In particular,
a resol type phenol resins are preferable. The purity thereof may
adequately be controlled and selected depending on purposes. However, it
is preferable that the organic compound be used which contains metals each
in an amount of 5 ppm or less when highly pure silicon carbide powder is
desirable.
A ratio of carbon to silicon (hereinafter, referred to as a C/Si ratio) is
defined by means of elementary analysis of a carbide intermediate obtained
by carbonizing the mixture at 1,000.degree. C. The free carbon in the
silicon carbide produced stoichometrically would become 0% when the C/Si
ratio is 3.0. However, the free carbon is actually generated at a lower
C/Si ratio due to vaporization of SiO gas generated simultaneously. It is
important to determine the blending ratio previously such that the amount
of the free carbon in the silicon carbide powder produced does not become
inadequate for the sintered body production purpose. Generation of the
free carbon can be inhibited typically at the C/Si ratio of 2.0 to 2.5 for
the baking at around 1 atm and 1,600.degree. C. or higher. This range may
be used advantageously. The C/Si ratio of 2.5 or higher increases
remarkably the free carbon, which has an effect of inhibiting growth of
the particles. With this respect, the C/Si ratio may be selected
adequately depending on purposes of particle formation. It is, however,
noted that the C/Si ratio for obtaining the pure silicon carbide will vary
when the baking is made at a high or low atmospheric pressure. In such a
case, it is not limited to the above mentioned range of the C/Si ratio.
Since free carbon has a very weak effect on sintering as compared with
that of carbon originated from a non-metal-based sintering additive which
covers silicon carbide powder used in the present invention, the free
carbon may be negligible.
In the present invention, a mixture of the silicon source and the organic
compound may be solidified to form a solid product, if necessary, to
obtain a solid product of a homogenous mixture of the silicon source and
the organic compound that produces carbon upon heating. The solidification
may be made by means of cross-linking upon heating, curing with a curing
catalyst, and by an electron beam or a radiation. The curing catalyst may
be selected adequately depending on the carbon source used. When the
carbon source is a phenol or furan resin, the curing agent may be acids
such as toluene sulfonic acid, toluene carboxylic acid, acetic acid,
oxalic acid, hydrochloric acid, and sulfuric acid, or amine such as
hexamine.
The solid product of the raw material mixture may be carbonized by heating,
if necessary. This is achieved by means of heating the solid product in a
non-oxidizing atmosphere of nitrogen or argon at a temperature of
800.degree. C. to 1,000.degree. C. for 30 to 120 minutes.
The silicon carbide is generated when the resultant carbonized compound is
heated in the non-oxidizing atmosphere of argon or the like at a
temperature between 1,350.degree. C. and 2,000.degree. C., both inclusive.
The time and temperature of baking may be selected adequately depending on
the desired properties including a particle diameter or the like. For more
effective production, however, it is preferable that the baking be made a
temperature between 1,600.degree. C. and 1,900.degree. C.
If powder of higher purity is required, the impurities are further removed
by means of heat treatment at a temperature from 2,000.degree. C. to
2,100.degree. C. for 5-20 minutes during the above mentioned baking.
As a process for producing silicon carbide powder of particularly high
purity, a process for producing a monocrystal described in Japanese Patent
Application No. 7-241856 filed by the present applicant may be used for
producing raw material powder. More specifically, this is a process for
making silicon carbide powder of high purity characterized by comprising a
silicon carbide production step for producing silicon carbide powder by
means of heating and baking in a non-oxidizing atmosphere a homogenous
mixture of a silicon source comprising one or more selected from
tetraalkoxysilane and polymers of tetraalkoxysilane of high purity and a
carbon source comprising an organic compound of high purity that produces
carbon upon heating; and a post-treatment step in which the resultant
silicon carbide powder is kept at a temperature from equal to or higher
than 1,700.degree. C. to lower than 2,000.degree. C. and is heated, at
least once, to a temperature between 2,000.degree. C. and 2,100.degree. C.
for 5 to 20 minutes while the temperature between 1,700.degree. C. and
2,000.degree. C. is kept, to obtain silicon carbide powder having a
content of each impurity of 0.5 ppm or less through the above two steps.
The non-metal-based sintering additives used in making the silicon carbide
sintered body of the present invention to be mixed with the above
mentioned silicon carbide powder may be a substance called the carbon
source that produce carbon upon heating. Examples of the carbon sources
used include organic compounds that produce carbon upon heating and the
silicon carbide powder (particle diameter: 0.01-1 .mu.m) covered with
these compound. The organic compounds are more preferable in view of
effects.
Specific examples of the organic compounds that produce carbon upon heating
include coal tar pitch, pitch tar, phenol resins, furan resins, epoxy
resins, phenoxy resins, and various saccharides including monosaccharides
such as glucose, oligosaccharides such as sucrose, and polysaccharides
such as cellulose and starch, having a high residual carbon ratio.
Suitably used are those in the liquid form at an ordinary temperature,
those to be dissolved into solvents, those to be softened or liquefied
upon heating such as thermoplastic or thermomelting materials. Of these,
the phenol resins, particularly resol type phenol resins are desirable
with which a molded product of a high strength can be obtained. The
above-described organic compounds generate inorganic carbon compounds such
as carbon black and graphite when the organic compounds are heated in the
system. The inorganic compounds generated are considered to be effective
as sintering additives. When inorganic carbon compounds such as carbon
black and graphite as sintering additives are added directly to the
sintering system, the effect of the present invention cannot be achieved.
In the present invention, it is preferable that when the mixture of the
silicon carbide powder and a non-metal-based sintering additive are
prepared, the non-metal-based sintering additive is dissolved or dispersed
in a solvent. The solvent may be the one that is suitable for the compound
used as the non-metal-based sintering additives. More specifically, the
solvent may be a lower alcohol such as ethyl alcohol, ethyl ether, or
acetone for the phenol resin that is a suitable organic compound which
produces carbon upon heating. It is preferable that the non-metal-based
sintering additive and the solvent used have amounts of impurities as low
as possible.
Typically, the amount of the non-metal-based sintering additive to be mixed
with the silicon carbide powder is preferably adjusted to an amount of 10%
or less, preferably 2-5% by weight as carbon, though depending on the type
of the non-metal-based sintering additive used, because an excessively
small amount of the non-metal-based sintering additive prevents the
sintered body from being densified while an excessively large amount
increases the amount of free carbon contained in the sintered body, which
tends to inhibit densification. The amount may be determined in advance by
quantifying the amount of silica (silicon oxide) on the surface of the
silicon carbide powder with hydrofluoric acid and calculating a
stoichiometric amount sufficient to reduce it.
Here, the amount of addition as carbon as described above in the amount
determined in light of a residual carbon ratio (ratio of production of
carbon in a non-metal-based sintering additive) after the thermal
decomposition of the non-metal-based based sintering additive, where the
silica (SiO.sub.2) is reduced by the carbon originated from the
non-metal-based sintering additive according to the following equation:
SiO.sub.2 +3C.fwdarw.SiC+2CO
The silicon carbide sintered body of the present invention preferably
contains carbon atoms originated from silicon carbide contained in the
silicon carbide sintered body and originated from the non-metal-based
sintering additive, in an amount of more than 30% by weight and less than
40% by weight. When the sintered body does not contain any impurities, the
content of carbon atoms is theoretically 30% by weight. That is, if
impurities are contained in the sintered body in a high amount, the
content of carbon atoms in the sintered body becomes less than 30% by
weight. This is not preferable. It is also not preferable that the amount
of carbon atoms is more than 40% by weight, since the density of the
sintered body obtained becomes lower and various properties such as the
strength of and the resistance to oxidation of the sintered body are
deteriorated.
When the silicon carbide sintered body of the present invention is
prepared, the silicon carbide powder and the non-metal-based sintering
additive are mixed homogeneously first. As mentioned above, the phenol
resin which is a preferable non-metal-based sintering additive is
dissolved in a solvent such as ethyl alcohol to mix it well with the
silicon carbide powder. The mixing may be carried out by use of known
mixing means such as a mixer, a planetary ball mill or the like. It is
preferable that the mixing be carried out for 10-30 hours, particularly
16-24 hours. After throughout mixing, the solvent is removed at a
temperature compatible with physical properties of the solvent used, e.g.,
at a temperature of 50-60.degree. C. for ethyl alcohol described above.
The mixture evaporated to dryness and the resultant material is sieved to
obtain raw material powder of the mixture. It is necessary, from the
viewpoint of purification to a higher degree, that a ball mill and balls
are made of a synthetic resin containing little or no metal. For drying, a
granulator such as a spray dryer may be used.
The sintering step which is an essential step in the production process for
producing a sintered body of the present invention is a step in which the
powdery mixture or a molded body of the powdery mixture obtained in a
molding step described below is placed in a mold in a non-oxidizing
atmosphere at a temperature of 2,000-2,400.degree. C. and a pressure of
300-700 kgf/cm.sup.2 for hot pressing.
It is preferable that a material made of graphite be used at apart of or
whole mold or that a sheet of Teflon be intervened between the molded
product and metal portions of the mold in the mold so as to prevent direct
contact therebetween from the viewpoint of the purity of the resultant
sintered body.
In the present invention, the hot pressing may be carried out at a pressure
of 300 to 700 kgf/cm.sup.2. In particular, it is necessary to select
components for the hot pressing used herein such as a die and a punch
having a good pressure resistance when a pressure of 400 kgf/cm.sup.2 or
more is applied.
Now, the sintering step is described in detail. It is preferable that the
impurities be removed well and the carbon of the non-metal-based sintering
additive be carbonized completely by heating and increasing temperature
under the following conditions before the hot press step for producing the
sintered body by the hot press treatment.
More specifically, it is preferable that a temperature increase step with
the following two stages be carried out. First, the inside of a furnace is
heated gradually from room temperature to 700.degree. C. in vacuum. The
temperature may be increased continuously up to 700.degree. C. when it is
difficult to control the temperature in the furnace. Preferably, the
inside of the furnace is adjusted at 10.sup.-4 torr and the temperature is
increased gradually from room temperature to 200.degree. C., at which the
temperature of the furnace is kept for a predetermined time. The
temperature is further increased gradually to 700.degree. C. The furnace
is kept for a certain time at a temperature around 700.degree. C. In this
first stage of the rise of temperature, adsorbed water and organic
solvents are eliminated and carbonization proceeds as a result of thermal
decomposition of the non-metal-based sintering additives. The time during
which the temperature is kept around 200.degree. C. or around 700.degree.
C. is selected to be an adequate range in accordance with the size of the
sintered body. The time when decrease of degree of vacuum becomes slight
to some extent may be a rough measure to determine whether the holding
time is enough. If the sintered body is abruptly heated at this timing,
the removal of the impurities and carbonization of the carbon of the
non-metal-based sintering additive are not performed enough. This may
cause cracking and voids in the sintered body.
As an example, a sample of 5-10 g is heated gradually at 10.sup.-4 torr
from room temperature to 200.degree. C. and is kept at that temperature
for about 30 minutes. Subsequently, the sample is heated gradually to
700.degree. C. The time taken to heat the sample from room temperature to
700.degree. C. is from 6 to 10 hours, preferably about 8 hours. It is
preferable that the sample be held at a temperature around 700.degree. C.
for 2-5 hours.
In vacuum, the sample is heated from 700.degree. C. to 1,500.degree. C.
over 6-9 hour periods under the above mentioned conditions and is held at
1,500.degree. C. for 1-5 hours. It is assumed that the silicon dioxide and
silicon oxide are reduced during this process. It is important to complete
this reduction in order to remove oxygen bonded to silicon. It is thus
necessary that the sample is kept at 1,500.degree. C. until generation of
carbon monoxide is finished that is a by-product of this reducing
reaction, i.e., until decrease of the degree of vacuum becomes slight and
the degree of vacuum is recovered to that achieved at around 1,300.degree.
C., which is the temperature before the beginning of the reducing
reaction. The reduction in this second stage of the rise of temperature
contributes to the removal of silicon dioxide that adheres to the surface
of the silicon carbide powder to inhibit densification and cause growth of
larger particles. Gases which are generated in this reduction process
contain SiO and CO, and are accompanied by elements of impurities. The
gases generated are continuously exhausted into a reactor by the use of a
vacuum pump and are removed from the silicon carbide. Accordingly, it is
preferable that the temperature be kept well from the viewpoint of
achieving a higher purity.
It is preferable that the hot press be carried out at a high temperature
after completion of the temperature increasing stages. The sintering
starts when the temperature reaches at a temperature higher than
1,500.degree. C. In this event, application of the pressure is started and
the pressure is increased to approximately 300-700 kgf/cm.sup.2 in order
to inhibit abnormal growth of the particles. Thereafter, an inert gas is
introduced into the furnace to provide a non-oxidizing atmosphere in the
furnace. The inert gas used may be nitrogen or argon. It is, however,
preferable that argon be used because it is not reactive at a high
temperature.
After the inside of the furnace is brought to a non-oxidizing atmosphere,
heat and pressure are applied to achieve a temperature of 2,000.degree. C.
to 2,400.degree. C., and a pressure of 300 to 700 kgf/cm.sup.2. The
pressure upon pressing may be selected depending on the particle diameter
of the raw material powder. With a smaller particle diameter of the raw
material powder, it is possible to obtain a favorable sintered body at a
relatively low pressure to be applied. The rise of temperature from
1,500.degree. C. to a highest temperature of 2,000-2,400.degree. C. is
carried out over 2-4 hours, during which the sintering proceeds rapidly at
around 1,850-1,900.degree. C. The temperature is kept for 1-3 hours at
this highest temperature to complete the sintering.
In this event, the densification is not sufficient when the highest
temperature is lower than 2,000.degree. C. On the other hand, the highest
temperature of higher than 2,400.degree. C. is not preferable because the
powder or the raw material of the sintered body tends to sublimate or be
decomposed. The densification is not sufficient when the pressure is lower
than 300 kgf/cm.sup.2. On the other hand, the pressure of higher than 700
kgf/cm.sup.2 is not preferable from the viewpoint of efficiency of
production because such a high pressure may cause damage of a mold made
of, for example, graphite.
In this sintering step, it is preferable from the viewpoint of keeping
purity of the resultant sintered body that a graphite material of high
purity be used for the graphite mold and insulating materials of a heating
furnace. The graphite material used is the one subjected to purification
treatment. More specifically, it is preferable to use the graphite
material that is previously baked well at a temperature equal to or higher
than 2,500.degree. C. and that produces less or no impurities at a
sintering temperature. In addition, it is also preferable to use the inert
gas which is highly purified one containing less or no impurities.
In the present invention, the above-mentioned sintering step provides a
silicon carbide sintered body having superior properties. A following
molding step may be carried out prior to the sintering step in light of
the densification of the final sintered body. The molding step that may be
carried out prior to the sintering step is described below. In this event,
the molding step is a step in which a raw material powder obtained by
homogeneously mixing the silicon carbide powder and a non-metal-based
sintering additive is placed in a mold and is heated under pressure at a
temperature ranging from 80 to 300.degree. C. for 5 to 60 minutes to
prepare a molded body. It is preferable that the raw material powder be
filled in the mold as dense as possible from the viewpoint of the
densification of the final sintered body. This molding step allows to make
bulky powder compact previously in filling the sample for the hot
pressing. Accordingly, this facilitates production of a thick sintered
body by repeating this molding step.
At a heating temperature ranging from 80 to 300.degree. C., preferably from
120 to 140.degree. C. depending on properties of the non-metal-based
sintering additive used, and under a pressure ranging from 60 to 100
kgf/cm.sup.2, the raw material powder filled is pressed to provide a
density of 1.5 g/cm.sup.3 or higher, preferably 1.9 g/cm.sup.3 or higher,
and is kept under pressure for 5 to 60 minutes, preferably 20 to 40
minutes to produce a molded body made of the raw material powder. The
smaller the average particle diameter is, the more it becomes difficult to
increase the density of the molded body. For achieving a higher density of
the molded body, it is preferable to use vibration packing for placing the
powder in the mold. More specifically, the density is preferably 1.8
g/cm.sup.3 or higher for the powder having the average particle diameter
of about 1 .mu.m, and 1.5 g/cm.sup.3 for the powder having the average
particle diameter of about 0.5 .mu.m. The density of lower than 1.8
g/cm.sup.3 and 1.5 g/cm.sup.3 for the particle diameters of 1 .mu.m and
0.5 .mu.m, respectively, makes it difficult to increase the density of the
final sintered body.
The molded body may be cut so as to fit in a hot press mold before it is
subjected to the subsequent sintering step. The molded body is placed in
the mold in a non-oxidizing atmosphere at a temperature of 2,000 to
2,400.degree. C. and a pressure of 300 to 700 kgf/cm.sup.2. The molded
body is thus subjected to the sintering step where it is hot pressed to
obtain a highly pure and highly dense silicon carbide sintered body.
The silicon carbide sintered body produced in the manner described above is
sufficiently dense with a density of 2.9 g/cm.sup.3 or higher. The density
of the resultant sintered body of lower than 2.9 g/cm.sup.3 deteriorates
mechanical properties such as bending strength and fracture strength as
well as electrical properties, enlarges the particles, and aggravates
contamination. It is more preferable that the silicon carbide sintered
body have a density of 3.0 g/cm.sup.3 or higher.
When the resultant sintered body is porous, such problems arise that heat
resistance, oxidation resistance, chemical resistance and mechanical
strength thereof are low, and that the sintered body is difficult to be
washed, that fine cracks in the sintered body are caused to form small
pieces thereof being contaminants, and that the sintered body has a gas
permeability. Consequently, these problems result in limited applications
of the sintered body.
The silicon carbide sintered body obtained in the present invention has a
content in total of impurities of 5 ppm or less, preferably 3 ppm or less,
and more preferably 1 ppm or less. The impurity contents obtained by means
of chemical analyses only have a meaning of reference values from the
viewpoint of applying it to the semiconductor industrial field. In
practice, assessment varies depending on whether the impurities are
distributed uniformly or localized. Accordingly, those skilled in the art
typically assess through various means to what extent the impurities
contaminate wafers under predetermined heating conditions by using a
practical device. According to a production process comprising a
carbonizing step by heating, in a non-oxidizing atmosphere, a solid
product obtained by mixing homogeneously a liquid silicon compound, a
non-metal-based sintering additive, and a polymerization or cross-linking
catalyst, and the subsequent baking step thereof in the non-oxidizing
atmosphere, it is possible to make the contents in total of the impurities
other than silicon, carbon, and oxygen in the silicon carbide sintered
body be 1 ppm or less.
By studying favorable properties of the silicon carbide sintered body
obtained in the present invention, it is preferable that the sintered body
have a bending strength at room temperature of 500 to 650 kgf/mm.sup.2, a
bending strength at 1,500.degree. C. of 550 to 800 kgf/mm.sup.2, a Young's
modulus of 3.5.times.10.sup.4 to 4.5.times.10.sup.4, a Vickers hardness of
2,000 kgf/mm.sup.2 or higher, a Poisson's ratio of 0.14 to 0.21, a
coefficient of thermal expansion of 3.8.times.10.sup.-6 to
4.2.times.10.sup.-6 (.degree. C..sup.-1), a thermal conductivity of 150
W/m.cndot.k or higher, a specific heat of 0.15 to 0.18
cal/g.cndot..degree. C., a thermal shock resistance of 500 to 700
.DELTA.T.degree. C., and a resistivity of 0.01 .OMEGA..cndot.cm or higher.
The sintered body obtained by means of the production process described
above may be subjected to treatments such as machining, polishing, and
washing in accordance with its objects for use. The sintered body of the
present invention may be produced by forming a cylindrical sample
(sintered body) by hot press and by slicing it in the radial direction.
For this machining, electrical discharge machining is suitably used. Then,
the machined sintered body is used for parts and components for
manufacturing semiconductors and electronic information equipment.
Representative examples of the semiconductor manufacturing device where the
parts and components made of the sintered body of the present invention
are used include an exposure equipment, resist processing equipment, dry
etching equipment, cleaning equipment, heat treatment equipment, ion
implanter, CVD equipment, PVD equipment, and dicing equipment. Examples of
the parts and components include plasma electrodes for the dry etching
equipment, protection ring (focus rings), slit component (aperture) for
the ion implanter, protection plate for an ion generation unit and mass
spectrometer, and a dummy wafer used in wafer treatment in the heat
treatment equipment or the CVD equipment and a heater used in the heat
treatment equipment, CVD equipment or PVD equipment, more specifically, a
heater for heating a wafer directly at the bottom thereof.
Examples of the parts and components for the electronic information
equipment include a disk base for a hard disk device and thin film
magnetic head base. In addition, sputtering targets for use in forming a
thin film on surfaces of magneto-optical disks and other sliding surfaces
are also included in these parts and components.
The parts made of the sintered body of the present invention may be used
for a reflection mirror for synchrotron radiation beam or laser beam.
In the production process of the present invention, there is no specific
limitation to production apparatuses as long as the above-mentioned
heating conditions of the present invention are satisfied. In light of the
pressure resistance of the mold used for the sintering, known reactors and
heating furnaces may be used.
It is preferable that the purity of the silicon carbide powder which is a
raw material powder, of the silicon and carbon sources for use in
producing the raw material powder, and the inert gas used for providing
the non-oxidizing atmosphere be 1 ppm or less in contents of each impurity
element. However, it is not necessary to limit to the above range as long
as it is in an allowable range of purification in the heating and
sintering steps The term "impurity element" used herein means group 1 to
group 16 elements in the periodic table of revised version of IUPAC
inorganic chemistry nomenclature in 1989 that have an atomic number of not
smaller than 3 except for those having an atomic number of 6 to 8 and 14
to 16.
EXAMPLES
Though the present invention is described specifically in conjunction with
a set of examples, it is understood that the present invention is not
limited to those examples as long as it does not outside the scope of the
present invention.
Example 1
Production of Molded Body
90 g of a silicon carbide powder of high purity (average particle diameter
of 1.1 .mu.m: silicon carbide powder containing silica of 1.5% by weight
and having an impurity content of 5 ppm or less obtained in accordance
with a production process filed as the above mentioned Japanese Patent
Application No. 7-241856) and a solution of 10 g of liquid resol type
phenol resin (residual carbon ratio after thermal decomposition: 50%) of
high purity having a water content of 20% dissolved in 150 g of ethanol
were agitated for 18 hours in a planetary ball mill and mixed with each
other sufficiently. Then, the mixture was heated to 50-60.degree. C. to
evaporate ethanol to dryness and was screened through a sieve of 500 .mu.m
to obtain homogenous raw material powder of silicon carbide. 8.5 g of this
raw material powder was filled in a mold of 30 mm.phi. and was pressed at
130.degree. C. for 20 minutes to obtain a molded body having a density of
2.1 g/cm.sup.3.
Production of Sintered Body
This molded body was placed in a graphite mold and was subjected to hot
pressing under the following conditions. A hot press machine used was a
high frequency induction heating 10-t hot press.
(Conditions for Sintering Step)
The temperature was increased from room temperature to 700.degree. C. under
a vacuum condition of between 10.sup.-5 and 10.sup.-4 torr over a 6-hour
period and it was kept at that temperature for 5 hours (first stage of
temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. for additional 3 hours. It was then kept at that
temperature for 1 hour (second stage of temperature increase).
Thereafter, it was pressed at a pressure of 500 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,200.degree. C. in an
argon atmosphere over a 3-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 3.15 g/cm.sup.3, a Vickers
hardness of 2,300 kgf/mm.sup.2, and an electrical resistivity of 0.02
.OMEGA..cndot.cm. These properties are shown in Table 1 below. In
addition, concentrations of the impurities are given in Table 2 below.
Physical properties of the sintered body obtained in Example 1 were
measured precisely. As a result, it was found that the sintered body had,
other than the above mentioned properties, a bending strength of 570
kgf/mm.sup.2 at room temperature, a bending strength of 600 kgf/mm.sup.2
at 1,500.degree. C., a Young's modulus of 4.1.times.10.sup.4, a Poisson's
ratio of 0.15, a coefficient of thermal expansion of 3.9.times.10.sup.-6
.degree. C..sup.-1, a thermal conductivity of 200 W/m.cndot.k or higher, a
specific heat of 0.16 cal/g.cndot..degree. C., and a thermal shock
resistance of 530 .DELTA.T.degree. C., all of which fell within the above
mentioned preferable ranges.
Example 2
Production of Molded Body
A molded body having a density of 1.85 g/cm.sup.3 was obtained in a similar
manner to that in Example 1.
Production of Sintered Body
This molded body was placed in a graphite mold and was subjected to hot
pressing under the following conditions. A hot press machine used was the
same as the one used in Example 1.
(Conditions for Hot Pressing)
The temperature was increased from room temperature to 700.degree. C. under
a vacuum condition of between 10.sup.-5 and 10.sup.-4 torr over a 6-hour
period and it was kept at that temperature for 1 hour (first stage of
temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. over an additional 3-hour period. It was then kept
at that temperature for 1 hour (second stage of temperature increase).
Thereafter, it was pressed at a pressure of 500 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,100.degree. C. in an
argon atmosphere over a 3-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 3.09 g/cm.sup.3, a Vickers
hardness of 2,200 kgf/mm.sup.2, and an electrical resistivity of 1.0
.OMEGA..cndot.cm. These properties are shown in Table 1 below. In
addition, concentrations of the impurities are given in Table 2 below.
Example 3
Production of Molded Body
A molded body having a density of 1.5 g/cm.sup.3 was obtained in a similar
manner to that in Example 1.
Production of Sintered Body
This molded body was placed in a graphite mold and was subjected to hot
pressing under the following conditions. A hot press machine used was the
same as the one used in Example 1.
(Conditions for Hot Pressing)
The temperature was increased from room temperature to 700.degree. C. under
a vacuum condition of between 10.sup.-5 and 10.sup.31 4 torr over an
8-hour period and thereafter, it was kept at that temperature for 1 hour
(first stage of temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. over an additional 3-hour period. It was then kept
at that temperature for 4 hours (second stage of temperature increase).
Thereafter, it was pressed at a pressure of 500 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,200.degree. C. in an
argon atmosphere over a 4-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 3.18 g/cm.sup.3, a Vickers
hardness of 2,300 kgf/mm.sup.2, and an electrical resistivity of 0.03
.OMEGA..andgate.cm. These properties are shown in Table 1 below. In
addition, concentrations of the impurities are given in Table 2 below.
Example 4
Production of Molded Body
90 g of commercially available .beta.-type silicon carbide powder (produced
by H. C. Schtark Co., average particle diameter of 2 .mu.m; containing
silica of 3.0% by weight) and a solution of 10 g of liquid resol type
phenol resin of high purity having a water content of 20% dissolved in 150
g of ethanol were agitated for 18 hours in a planetary ball mill and mixed
with each other sufficiently. Then, the mixture was heated to
50-60.degree. C. to evaporate ethanol to dryness and was screened through
a sieve of 500 .mu.m to obtain homogenous raw material powder of silicon
carbide. 8.5 g of this raw material powder was filled in a mold of 30
mm.phi. and was pressed at 130.degree. C. for 20 minutes to obtain a
molded body having a density of 2.2 g/cm.sup.3.
Production of Sintered Body
This molded body was placed in a graphite mold and was subjected to hot
pressing under the following conditions. A hot press machine used was a
high frequency induction heating 10-t hot press.
(Conditions for Sintering Step)
The temperature was increased from room temperature to 700.degree. C. under
a vacuum condition of between 10.sup.-5 and 10.sup.-4 torr over a 6-hour
period and it was kept at that temperature for 5 hours (first stage of
temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. over an additional 3-hour period. It was then kept
at that temperature for 1 hour (second stage of temperature increase).
Thereafter, it was pressed at a pressure of 500 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,200.degree. C. in an
argon atmosphere over a 3-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 3.18 g/cm.sup.3, a Vickers
hardness of 2,300 kgf/mm.sup.2, and an electrical resistivity of 0.02
.OMEGA..cndot.cm. These properties are shown in Table 1 below.
Example 5
8.5 g of raw material powder of silicon carbide obtained in the same manner
as described in Example 1 was filled in the mold used in Example 1 and was
pressed. Then, the powder was filled directly in a graphite mold without
the molding step and was hot pressed under similar conditions as in
Example 1. The hot press machine used was the same as the one used in
Example 1.
(Conditions for Hot Pressing)
The temperature was increased from room temperature to 700.degree. C. under
a vacuum condition of between 10.sup.-5 and 10.sup.-4 torr over an 8-hour
period and it was kept at that temperature for 1 hour (first stage of
temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. for additional 3 hours. It was then kept at that
temperature for 4 hours (second stage of temperature increase).
Thereafter, it was pressed at a pressure of 500 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,200.degree. C. in an
argon atmosphere over a 4-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 3.05 g/cm.sup.3, a Vickers
hardness of 2,500 kgf/mm.sup.2, and an electrical resistivity of 0.03
.OMEGA..cndot.cm. These properties are shown in Table 1 below.
Comparative Example 1
Production of Molded Body
A molded body having a density of 2.0 g/cm.sup.3 was prepared in a similar
manner to that in Example 1.
Production of Sintered Body
This molded body was placed in a graphite mold and was subjected to hot
pressing under the following conditions. A hot press machine used was the
same as the one used in Example 1.
(Conditions for Hot Pressing)
The temperature was increased from a room temperature to 700.degree. C.
under a vacuum condition of between 10.sup.-5 and 10.sup.-4 torr over a
6-hour period and it was kept at that temperature for 1 hour (first stage
of temperature increase).
The temperature was increased from 700.degree. C. to 1,200.degree. C. under
vacuum over a 3-hour period and was further increased from 1,200.degree.
C. to 1,500.degree. C. over an additional 1.5-hour period. It was then
kept at that temperature for 1 hour (second stage of temperature
increase).
Thereafter, it was pressed at a pressure of 150 kgf/cm.sup.2 and the
temperature was increased from 1,500.degree. C. to 2,200.degree. C. in an
argon atmosphere over a 3-hour period. It was then kept at that
temperature for 1 hour (hot press step).
The resultant sintered body had a density of 2.45 g/cm.sup.3, a Vickers
hardness of 1,900 kgf/mm.sup.2, and an electrical resistivity of
1.times.10.sup.3 .OMEGA..cndot.cm. A number of voids were found in the
sintered body. These properties are shown in Table 1 below.
Example 6
Pitch tar was dissolved in ethanol and added in an amount of 25% by weight
to silicon carbide as the organic compound that produces carbon upon
heating to apply this organic compound to the surface of the silicon
carbide powder. Then, raw material powder of silicon carbide obtained in a
similar manner as in Example 5 was sintered under the same conditions as
in Example 5.
The resultant sintered body had a density of 3.05 g/cm.sup.3, a Vickers
hardness of 2,200 kgf/mm.sup.2, and an electrical resistivity of 0.1
.OMEGA..cndot.cm. These properties are shown in Table 1 below.
Comparative Example 2
Carbon black was added in an amount of 2% by weight to silicon carbide
powder as an inorganic carbon-based sintering additive and was dispersed
well along with the silicon carbide powder by using ethanol. The mixture
was mixed sufficiently and was then dried to obtain powder to be sintered.
The powder was hot pressed under the same conditions as in Example 5
without the molding step. The resultant sintered body had a density of
only 2.6 g/cm.sup.3, indicating that a sintered body having a sufficient
density could not be obtained.
Comparative Example 3
62.1 g of tetraethyl silicate, 30 g of liquid resol type phenol resin of
high purity having a water content of 20%, and 5 g of a 50% aqueous
solution of toluene sulfonic acid were used as a silicon source, a carbon
source, and a curing catalyst, respectively, and were blended so as to be
the C/Si ratio of 2.6. The mixture was then solidified, dried, carbonized,
and baked at 1,900.degree. C. in an argon atmosphere. As a result, raw
material powder of silicon carbide was obtained which had a particle
diameter of 1.5 .mu.m and contained 1.0% by weight of free carbon. The raw
material powder was sintered under the same conditions as in Example 5
without adding any sintering additives. The resultant sintered body had a
density of only 2.7 g/cm.sup.3, indicating that a sintered body having a
sufficient density could not be obtained.
Comparative Example 4
Boron carbide (B.sub.4 C) was added in an amount of 1% by weight to silicon
carbide powder as a metal-based sintering additive and was dispersed well
along with the silicon carbide powder by using ethanol. The mixture was
mixed sufficiently and was then dried to obtain powder to be sintered. The
powder was hot pressed under the same conditions as in Example 5 without
the molding step.
The resultant sintered body was dense with a density of 3.15 g/cm.sup.3 and
a Vickers hardness of 2,400 kgf/mm.sup.2. However, the sintered body had
an electrical resistivity of 10.sup.5 .OMEGA..cndot.cm and was not
electrically conductive. This sintered body was immersed in a mixture of
hydrofluoric acid and nitric acid (1:1) and was heated under pressure in a
sealed container. 2 hours later, the purity of the liquid in the container
was analyzed. As a result, boron of more than 1,000 ppm was detected.
TABLE 1
__________________________________________________________________________
Physical Properties of Sintered Bodies
Comparative
Comparative
Comparative
Comparative
Example 1 Example 2
Example 3
Example 4
Example 5
Example 6
Example 1
Example 2
Examp
Example
__________________________________________________________________________
4
Density
3.15 3.09 3.18 3.18 3.05 3.05 2.45 2.6 2.7 3.15
(g/cm.sup.3)
Electrical
0.02
1,000
10.sup.4
10.su
10.sup.5
resistivity
(.OMEGA. .multidot. cm)
Hardness
2,200
1,900
1,900
1
2,400
(kgf/mm.sup.2)
Thermal
200
180
192 140
195 130
50 70 80 80
Conductivity
(W/m .multidot. k)
__________________________________________________________________________
TABLE 2
______________________________________
Impurity Concentration in Sintered Bodies (unit: ppm)
Comparative
Example 1 Example 2
Example 3
Example 1
______________________________________
B 0.00 0.00 0.00 0.00
Al 0.02
0.02
0.02
Na 0.03
0.02
0.01
K 0.00
0.00
Mg 0.05
0.03
0.05
Ti 0.02
0.01
0.01
Cr 0.00
0.00
0.00
Fe 0.03
0.03
0.05
Ni 0.01
0.01
0.01
Co 0.03
0.02
0.02
W 0.00
0.00
Cu 0.00
0.01
0.01
______________________________________
As apparent from Examples and Comparative Examples in Table 1 above, the
silicon carbide sintered bodies obtained in Examples 1 through 6 according
to the process of the present invention are found to be sintered bodies of
high density each having a sufficient density which can be used
advantageously for various applications.
As apparent from Table 2, the sintered bodies of Examples 1 through 3 made
of the silicon carbide powder used as a raw material that is obtained by
purification treatment have a significantly low content of impurities and
good properties suitable for parts and components of semiconductor
manufacturing equipment and electronic information equipment, in light of
the physical properties thereof.
On the other hand, the sintered body in Comparative Example 1 where the hot
press was carried out at a low pressure and the sintered body in
Comparative Example 3 where no sintering additive was added had low
densities and were found to have a number of voids. The sintered body in
Comparative Example 4 where a metal-based sintering additive was used had
a high electrical resistivity. These sintered bodies were apparently not
suitable for parts and components of semiconductor manufacturing equipment
and electronic information equipment.
According to the present invention, it is possible to obtain a silicon
carbide sintered body of high quality which has a high density, a high
purity, a high electrical conductivity, and a high thermal conductivity,
which cannot be obtained through conventional processes. Accordingly, the
silicon carbide sintered body of the present invention are useful
materials in various fields including semiconductor industry, electronic
information equipment industry, and the like.
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